Conventionally, in this type of thermometer, a temperature sensor installed outdoors or indoors is connected by a wire to a display installed indoors or outdoors. In getting this conductor wire through, use has generally been made of a hole formed in a wall or a clearance in a window frame, but there has been a problem that it is very difficult to provide such hole or to cut off the communication between the interior of the room and the open air if a clearance is provided.